The present invention relates generally to optical switching networks and more particularly to an apparatus and method for reducing the number of control elements for crosstalk reduction devices in an optical switching network.
This invention was made with Government support under Agreement No. MDA972-95-3-0027 awarded by ARPA. The Government has certain rights in the invention.
Optical communication systems use optical signals to convey information over an optical transmission medium, typically a waveguiding medium such as optical fiber. The usable transmission capacity of a given waveguiding medium can be substantially increased by the use of wavelength division multiplexing (WDM) techniques. WDM is a method for increasing the capacity of an optical transmission system by simultaneously operating a plurality of optical signals at different wavelengths over one medium and can be used for both long-haul transmission systems and small local area networks. With WDM, different multiplexed optical signals can be transmitted at different wavelengths, referred to as channel wavelengths, through the same transmission medium.
The extensive use of WDM techniques necessitates the use of speedy interconnected elements such as optical switching modules. The switching modules, typically made using lithium niobate (LiNbO3), a ferroelectric material, are necessary for the effective routing and control of optical signals from many different paths. However, as the number of interconnected elements and waveguides increase, crosstalk among signals in the waveguides and optical interconnectors is increasingly a problem.
Generally in an optical network, light beams are modulated in a digital or analog fashion and are used as optical carriers of information. There are many reasons why light beams or optical carriers are preferred in these applications. For example, as the data rate required of such channels increases, the high optical frequencies provide a tremendous improvement in available bandwidth over conventional electrical channels such as formed by wires or coaxial cables. In addition, the energy required to drive and carry high bandwidth signals can be reduced at optical frequencies. Further, optical channels can be packed closely and even intersect in space.
Although optical switches have improved dramatically in the last few years, they do not function as perfect digital switches. That is, there is a certain amount of leakage inherently associated with the waveguide structure of the switch. For example, in a typical 1xc3x972 switch with one input port routed to one of two output ports, there may be some signal leakage of the active signal at the undesired output port which is characterized as crosstalk.
In one example of an optical switching network, arbitrary connections between N input channels to any of M output channels can be accomplished by a tree architecture. Although denoted xe2x80x9cchannels,xe2x80x9d the channels may refer to processors in a multiprocessor environment or fiber optic channels or the like. If desired, any electronic information at the input channels can be modulated on optical carriers and reconverted to electronic information at the output channels to emulate any electronic network. Optionally, modulation in the optical domain can be maintained to provide a ready interface to other optical interconnect schemes.
Referring to FIG. 1, there is shown, for illustration purposes, a 12xc3x976 tree architecture 10 coupling twelve input channels 65 with six output channels 75. A 1xc3x976 fan-out tree 15 is formed for each of the twelve input channels and a 12xc3x971 fan-in tree 20 is formed for each of the six output channels. As can be seen, each stage of the fan-out tree 15 is assembled with active 1xc3x972 switches 35. Similarly, each stage of the fan-in tree 20 is assembled with 2xc3x971 switches 45. Each 1xc3x972 switch is composed of one input port and two output ports and each 2xc3x971 switch is composed of two input ports and one output port. The fan-out tree 15 is composed of successive stages of 1xc3x972 switches which act as demultiplexers for each input channel. It should be apparent that a unique path for each of the 12xc3x976 possible connections between the input channels and output channels exists in the network of FIG. 1. Control signals (not shown) are also coupled to each stage of active switches to control the output of the 1xc3x972 switches and 2xc3x971 switches. Typically, to minimize the real estate taken up by control signals, a xe2x80x9cgangedxe2x80x9d approach is used to control each stage of switches. That is, all switches in the same stage of a given tree are switched by the same control signal. As will be described in detail below, the intersection of the fan-out tree and the fan-in tree is an advantageous location for the placement of crosstalk reduction devices 55 since the intersection serves as a possible opportunity for any crosstalk signals to combine with active signal paths.
Generally, an Nxc3x97M network arranged in a tree architecture exhibits [LogX N]+[LogX M] stages wherein X represents the number of output ports on each switch (2 in our example) within the topology and wherein the expression [Y] represents an integer greater than or equal to the argument Y. For instance, a 12xc3x976 tree architecture comprised of 1xc3x972 and 2xc3x971 switches exhibits [Log2 12]+[Log2 6] stages, for a total of 7 stages. Additionally, there are generally Nxc2x7M interconnection sites between the respective N 1xc3x97M fan-out trees and the respective M Nxc3x971 fan-in trees for insertion of crosstalk reduction devices. In the 12xc3x976 tree architecture of FIG. 1, there are 72 crosstalk reduction devices 55. Conventionally, for the Nxc2x7M crosstalk reduction devices, Nxc2x7M control voltage signals are necessary to individually control the crosstalk reduction devices. In the example of a 12xc3x976 tree architecture, 72 individual control voltage signals are necessary to control the crosstalk reduction devices.
Referring to FIG. 2, an illustration of crosstalk propagation that can result from a xe2x80x9cgangedxe2x80x9d approach to controlling a stage of switches is described using an exemplary embodiment of a 1xc3x9732 fan-out tree. The nodes on the figure are schematic representations of 1xc3x972 switches in a given 1xc3x9732 fan-out tree 15 for a specific input channel. At each switch, the active signal is routed to the desired output port. However, a xe2x80x9cknocked downxe2x80x9d version of the active signal is transmitted to the other output port because of leakage and is termed level 1 crosstalk. As this crosstalk propagates to the second stage of switches, the crosstalk is xe2x80x9cknocked downxe2x80x9d another level. That is, any leakage at a succeeding stage weakens the initial crosstalk signal. Referring again to FIG. 2, the double arrowed path shows the desired output of the fan-out tree 15 from one input channel denoting the active signal path. The single arrowed path shows the path of any level 1 crosstalk. At node 101, the active signal is routed to one of the output ports (arbitrarily shown in the Figure as an upward path and denoted by double arrows). At node 102, the knocked down crosstalk signal appears as level 1 crosstalk (shown in the Figure as a downward path and denoted by single arrows). However, because of the ganged approach to control signals at each stage, level 1 crosstalk is propagated to node 103. Similarly, nodes 104, 105 and 106 experience level 1 crosstalk because of the ganged approach to control signals. In contrast to the propagation of level 1 crosstalk, at the unintended output port at node 102, the level 1 crosstalk is xe2x80x9cknocked downxe2x80x9d a level to produce level 2 crosstalk at node 107. It should be apparent that nodes 111, 121, 131, 141 and 106 will have level 1 crosstalk appearing at the output of the fan-out tree 15 for a particular set of control signals for a particular active signal. Node 151 will have the active signal appearing at the node. All other nodes will have either a level 2 or a higher level crosstalk appearing at the node. Any other crosstalk (i.e. level 2 crosstalk, level 3 crosstalk, etc.) is acceptable since the crosstalk signals are sufficiently weakened by being knocked down at more than one switch.
With the proper placement and control of crosstalk reduction devices at the intersection of the fan-out tree and the fan-in tree, the level 1 crosstalk may be blocked while the active signal is allowed to pass. Crosstalk reduction devices have only two states, either passing the optical signal or blocking the optical signal based on a control signal. It should be apparent that in the implementation of FIG. 1, a total of 72 (12xc2x76) crosstalk reduction devices are required. Generally, a total of Nxc2x7M crosstalk reduction devices are required, each with its own control signal. For any large implementation of a tree architecture network (for example, 16xc3x9716), the number of control voltage signals for the crosstalk reduction devices can impose significant burdens on chip real estate and unduly complicate wiring.
Accordingly, there is a need for minimizing the number of control elements for crosstalk reduction devices at the intersection of the fan-out trees and the fan-in trees.
The invention is an apparatus and method for reducing the number of control signals for crosstalk reduction devices. Prior art implementations relying upon localized crosstalk reduction have generally required Nxc2x7M control signals. In the present invention, the number of control signals necessary for the control of crosstalk reduction devices is significantly reduced compared to the prior art and will be shown to be the smaller of N or M (for an arbitrary Nxc3x97M switching module).
The invention relates to the number of control elements for crosstalk reduction devices at the intersection of the fan-out trees and the fan-in trees for a tree architecture. The crosstalk reduction devices are partitioned into two mutually exclusive groups based on the levels of crosstalk propagated to the outputs of the fan-out tree. A pattern of crosstalk propagation is derived by assigning binary destination addresses to each switch within the fan-out trees. By successively changing the bits within the destination address of the active signal, two mutually exclusive groups of crosstalk reduction devices result. Therefore, for each input channel of the switching module, one control voltage signal is sufficient to control the operation of the crosstalk reduction devices. A similar approach can be taken for each output channel indicating that one control signal is sufficient for the proper operation of the crosstalk reduction devices associated with that output channel. Therefore, for an Nxc3x97M tree architecture, the number of control signals required for the operation of the crosstalk reduction devices is the lesser of N or M.